B.R. and W.A.D. thank Copernicus Astronomical Center in Warsaw and Istituto Nazionale di Fisica Nucleare, respectively, for the hospitality. This research was supported by Ministero dell'Università e Ricerca Scientifica (MURST) through grant 40% 1998 and by the Polish grant KBN-2P03D-014.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999